The present invention relates generally to electrically-induced osteogenesis, and specifically to an improved method and apparatus for implementing the method of Direct Current stimulated osteogenesis.
The use of Direct Current of from between 5 and 20.mu. amperes applied to a cathode inserted into a fracture site and an anode taped to the skin location near the cathode implantation is well-known in the art and is discussed in detail in U.S. Pat. No. 3,842,841, issued to Brighton, et al., on Oct. 22, 1974. The insertion of a cathode into the fracture site and the providing of a small current flow from the cathode to an anode taped on the skin has been found quite effective in the stimulation of bone growth at the tissue site surrounding the cathode. As reported in the Journal of Bone and Joint Surgery. Volume 62-A, No 1, pages 2-13, in January of 1981: "A Multi-Center Study of the Treatment of Non-Union With Constant Direct Current", a success rate of around 80 per cent was obtained in stimulating bone growth among patients having broken bones which had not mended over a substantial period of time.
In an abstract entitled "Electrically-Induced Osteogenesis: Relationship of Current Density to Quantity of Bone Formed", by Brighton, et al. published with the 24th Annual ORS (Orthopaedic Research Society), page 30, Feb. 21-23, 1978, it was found that the size of the "port" (the exposed cathode/tissue interface) should be at least 0.11 mm.sup.2 in area. However, the study also indicated that a cathode with multiple ports can distribute a much higher amount of Direct Current into a tissue site and stimulate a larger amount of bone growth therein. The above references are herein incorporated by reference.
Currently, systems are marketed by ZIMMER.multidot.USA, of Warsaw, Ind., 46580, which provide a constant current (.+-.5%) power which is supplied to either separate cathodes or a single cathode with multiple ports. FIG. 1 illustrates such prior art systems. Bone 10 has been fractured in the vicinity of tissue site 12, which is surrounded by living tissue 14 covered by skin 16. A cathode 18 having a single port 20 is inserted such that the port is in the vicinity where osteogenesis is to be stimulated. A surface anode 22 is placed in contact with the skin 16 and anode lead 24 and the cathode 18 are electrically connected to a constant current power supply 26. In the preferred embodiment, since there is only a single port at the tissue site 12, the power supply would produce 20 .mu. amps to produce the maximum amount of bone growth at the tissue site without incurring necrosis at either the tissue site or the anode/skin interface.
Even though such prior art Direct Current systems have a demonstrated capability to induce bone growth in living tissue, the amount of bone growth appears to vary from animal to animal or case to case, even though the current level between the cathode and the anode has been maintained at the believed optimum level of 20.mu. amps.